CHAPTER XVIII 

 MATERNAL INHERITANCE 



Theke is a kind of inheritance shown by eggs and 

 embryos, sometimes called maternal inheritance, that is 

 not the same as plastid inheritance, even although the lat- 

 ter is maternal in another sense. Nor is this so-called 

 maternal inheritance to be confused with cases of inheri- 

 tance in which all or some of the paternal chromosomes 

 fail to function, leaving the embryo at the mercy of its 

 maternal set alone. Nor should it be confused with sex- 

 linked inheritance w^here the son gets certain characters 

 only from the mother, because he gets his single sex- 

 chromosome from her. 



''True'' maternal inheritance relates to peculiarities 

 of the egg or larva that are due to materials already pres- 

 ent in the egg-cytoplasm when the egg is laid. For exam- 

 ple, if pigment is scattered in the egg, it may collect in 

 certain regions after fertilization, and produce color in 

 them, as does the yellow pigment in the egg of Cynthia, 

 studied by Conklin. In this ascidian, much of the yellow 

 pigment is carried at the moment of fertilization to that 

 part of the egg that later goes into the muscle of the tail. 

 If the sperm used to fertilize such an egg should come 

 from a species without pigment in the egg, the inheritance 

 of color of the young embryo would obviously be entirely 

 maternal. In cases like this one, the formed material, 

 or any substance producing such materials, is already 

 present in the cytoplasm, but whether it has always been 

 free from nuclear influence must be shown bv other tests. 

 In only one cross, vi^., in the silkworm, has a third genera- 

 tion been raised, and until this has been done in 

 others we cannot know whether we are dealing in them 

 with plastid or with deferred nuclear influence (''ma- 

 ternal inheritance''). 



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